Help sizing Cartier Juste un Clou pm based on Van Cleef Signature Perlee

[simurgh]

hi everyone! Thanks in advance for any help. I'd like to get a Cartier Juste un Clou pm. I have the Van Cleef perlee (in the old medium or regular size, since then they've added cm sizing, so I think the medium is now size 17). I like the fit of the perlee and would like to replicate it. Can anyone suggest what the best size of the JUC pm would be? thanks again!